To provide a more detailed explanation, our wireless service is provided by T-Mobile; unfortunately, as per CRTC regulations, T-Mobile is not permitted to build their own cell phone towers in Canada, however, they do have contracts with Canadian carriers in order to offer roaming to their customers. Essentially, every time you use your T-Mobile phone in Canada, T-Mobile has to pay the carrier whose signal you are using (e.g., Rogers, Bell etc).
As per T-Mobile policy roaming limits are 2 months. However, although we are not supposed to encourage the use of these lines primarily outside of the US, I have heard of a number of people using the service much longer than 2 months without the need to visit the US. The policy simply allows T-Mobile to suspend lines after 2 months should they feel someone is abusing the roaming feature. This is usually only for HEAVY data users, as this would mean a higher amount T-Mobile would be paying international carriers.